Systems and methods for pressure differential molecular spectroscopy of compressible fluids

Abstract

Described herein is a spectroscopic system and method for measuring and monitoring the chemical composition and/or impurity content of a sample or sample stream using absorption light spectroscopy. Specifically, in certain embodiments, this invention relates to the use of sample pressure variation to alter the magnitude of the absorption spectrum (e.g., wavelength-dependent signal) received for the sample, thereby obviating the need for a reference or ‘zero’ sample. Rather than use a reference or ‘zero’ sample, embodiments described herein obtain a spectrum/signal from a sample-containing cell at both a first pressure and a second (different) pressure.

What is claimed is: 
     
       1. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and 
 wherein the compressible fluid line is a natural gas pipeline stream and wherein the one or more constituents of the sample that are identified comprises one or more members selected from the group consisting of CH 4 , C 2 H 6 , C 3 H 8 , i-C 4 H 10 , n-C 4 H 10 , C 5 H 12 , and CO 2 . 
 
     
     
       2. The method of  claim 1 , wherein the first pressure is 0 psig. 
     
     
       3. The method of  claim 1 , wherein the first pressure and the second pressure differ by no less than 5 psig. 
     
     
       4. The method of  claim 3 , wherein the first pressure and the second pressure differ by no less than 10 psig. 
     
     
       5. The method of  claim 1 , where the ‘zero’ reference has a different composition than the sample. 
     
     
       6. The method of  claim 1 , wherein the sample fluid comprises a gas or compressible liquid. 
     
     
       7. The method of  claim 1 , wherein the compressible fluid flows through the fluid line continuously, semi-continuously, and/or intermittently. 
     
     
       8. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and wherein the compressible fluid line is an LNG line and wherein the one or more constituents of the sample that are identified comprises one or more members selected from the group consisting of CH 4 , C 2 H 6 , C 3 H 8 , i-C 4 H 10 , n-C 4 H 10 , and C 5 H 12 . 
 
     
     
       9. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and 
 wherein the compressible fluid line is an LPG line and wherein the one or more constituents of the sample that are identified comprises one or more members selected from the group consisting of C 2 H 6 , C 3 H 8 , i-C 4 H 10 , n-C 4 H 10 , and C 5 H 12 . 
 
     
     
       10. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and 
 wherein the compressible fluid line is an acid and/or sour gas line and wherein the one or more constituents of the sample that are identified comprises one or more members selected from the group consisting of H 2 S, CH 4 , C 2 H 6 , C 3 H 8 , i-C 4 H 10 , n-C 4 H 10 , C 5 H 12 , CO 2 , NH 3 , and H 2 O. 
 
     
     
       11. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and 
 wherein the compressible fluid line is a biomethane, syngas, and/or waste gas line and wherein the one or more constituents of the sample that are identified comprises one or more members selected from the group consisting of CO, CO 2 , CH 4 , H 2 S, and H 2 O. 
 
     
     
       12. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and 
 wherein the compressible fluid line is an olefins line and the one or more constituents of the sample that are identified comprises one or more members selected from the group consisting of CH 4 , C 2 H 6 , C 2 H 4 , C 3 H 8 , C 3 H 6 , n-C 4 H 10 , an alkane, an alkyne, an alkene, CO, CO 2 , H 2 S, H 2 , and O 2 . 
 
     
     
       13. A method for measuring chemical composition and/or trace impurities in a compressible fluid sample without the need for a ‘zero’ reference, the method comprising:
 obtaining a first spectrum for a sample cell containing a sample fluid from a spectroscopic system with the sample fluid at a first pressure in the sample cell; 
 obtaining a second spectrum for the sample cell containing the sample fluid from the spectroscopic system with the sample fluid at a second pressure in the sample cell, wherein the first pressure is different from the second pressure; 
 determining, by a processor of a computing device, a differential absorption spectrum for the sample using the first spectrum and the second spectrum; and 
 identifying, by the processor, one or more constituents of the sample, and/or identifying, by the processor, a concentration of one or more of the one or more constituents of the sample, using the differential absorption spectrum; 
 wherein the sample cell is a flow cell and the sample is drawn from a compressible fluid line, and 
 wherein the compressible fluid line is a flare stack gas line and the one or more constituents of the sample that are identified comprises a C1-C6 alkane.